Ramsey Electronics BS2C Manual de instrucciones descargar pdf (Pagina 5)

DCI2 • 5

The DTMF Decoder

The DTMF decoder, IC U3, a Holtek HT9170B, has the task of monitoring all

the input audio signals. When a valid DTMF signal is received, it sets a 4 bit

digital code on output pins 11, 12, 13, and 14.

Why a 4 bit code you may ask? Think back to your high school math class for a

moment when you learned about different base numbering systems. Humans

like to work in a base 10 system (decimal system, deci = 10) using the digits 1

through 10 (or 0 through 9 to be more precise). This is probably due to the 10

fingers and 10 toes we all learned to count on as kids. Computers on the other

hand (pardon the pun) like to work with a base 2 numbering system (binary

system, bi = 2) using digits 0 and 1; 0 for off / low and 1 for on / high. In order to

represent the 16 different possible tones (base 10) you need to have 4 bits in a

binary system 2

= 1111 (binary) = 16 (decimal). Wow… kind of got off on a

tangent there!

Internally, U3 has a set of counters that latch the 4 outputs depending on the

detected tones. The counters are referenced to a television color-burst crystal

operating at 3.579 MHz (X1) for rock solid measurements. Notice how C1

couples a bit of the X1 crystal frequency over to U1 (the micro-controller) to set

its timing as well.

The Micro-controller

U1 is a pre-programmed Freescale (Motorola) MC68HC908JK3 micro-

controller. This IC contains the software required to take the DTMF tone data

from U3 and process it while reading and writing to FLASH memory, generating

the feedback tones, monitoring the programming switch, and performing all the

output functions of the DC2. It does all this according to how you have

configured the DCI2. Don’t forget that it also handles all the configuration set-

up you do when you first start programming your DCI2. We won’t go into all the

functions of U1 because each of the configurations have different signal input

and output requirements. You can think of it as the traffic controller for the

DCI2. Quite a bit for one little part!

One other important signal line between U3 pin 15 and U1 pin 7 is the data

valid signal. Whenever U3 detects a valid DTMF signal it sends a high level

signal on this line to tell the microprocessor it needs to look at the 4 data lines

and make a decision what to do with the data. This signal line is also used by

U2 which will be discussed next.

The 4 to 16 Line Decoder

As you can see the DCI2 actually has two major “brains”. The DTMF decoder

U1 and the microprocessor U3. There is one more smart device we need to

talk about and that is U2, a 74HC154, a 4 line to 16 line decoder. If that sounds

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Ramsey Electronics BS2C Manual de instrucciones Pagina 5